More on the Tweed Volcano

I had the pleasure in obtaining a copy of a University of New England honours research thesis by Howden (2009) a week ago. For one thing, I'm pleased to see that there is still some research being conducted on the Tweed Volcano and Focal Peak Volcano, despite the state of our Country's university science faculties these days. Howden has put a great deal of effort into distinguishing between the mafic rocks of the volcano (basalts) including some detailed petrographic and geochemical analysis. One of the points of interest to me is the attempt to distinguish between the Blue Knob Basalt and Lismore Basalts, sadly, the work undertaken by Cotter (1998) was unavailable (lost to the world until recently) to her. This would have clarified some issues which were difficult to resolve in her thesis.

Previous authors such as Duggan & Mason (1978) noted that there appeared to be very little (if any) distinction between the Blue Knob and Lismore Basalts except for their apparent stratigraphic location. Duggan & Mason (1978) determined that the Blue Knob Basalt appeared to overlay the Nimbin Rhyolite and the Lismore Basalt under it. However, Duggan & Mason and other authors such as Smith & Houston (1995) suggested a possibility that the Blue Knob Basalt could actually be inter-collated with rhyolite flows indicating that it was possible that the basalts were really just occasionally interrupted by flows of the Nimbin Rhyolite.

Howden (2009) has through comprehensive geochemical and petrological study of the Lamington Volcanics demonstrated that the only way to distinguish between the two basalt units was on the basis of phenocryst size with the Blue Knob Basalt showing larger grains of plagioclase feldspar. In the absence of any other geochemical or petrological distinguishing characteristics this shows a very uninspiring difference between them, I would suggest, insufficient to say that they were in fact different.

Because of the absence of significant differentiating features it is likely that the Blue Knob Basalt is really just the Lismore Basalt which continued to erupt at various times with intervening periods of large rhyolitic eruptions of the Tweed Volcano. This means that this can be confirmed if flows of basaltic lava can be identified between rhyolite. In Queensland the equivalent of the Nimin Rhyolite, the Binna Burra Rhyolite shows intercollated flows of Hobwee Basalt (the equivalent of the Lismore Basalt). The plagioclase phenocryst grain size difference probably just reflects slightly different magma residence periods in the magma chamber becoming more obvious at the volcano became older. This is also demonstrated as the Hobwee Basalt in Queensland shows the upper flows have larger phenocrysts.

Slowly we are gaining a clearer picture of our present day landscape and the mechanisms that made it. Sometimes difference between the way we think they have occurred and they way we later find out seems quite minor, yet the implications are significant in understanding how the landscape actually behaves under the ground. The small areas of 'Blue Knob Basalt' were thought to be a last spurt of eruption of the Tweed Shied Volcano (either centred on present day Mount Warning, or other vents on the flanks of the volcano), I think that Howden (2009) has presented us with enough evidence how to say that the way the volcano formed included two different types of lavas (basalt and rhyolite) erupting at essentially the same time.

References/bibliography:

*Cotter, S. 1998. A Geochemical, Palaeomagnetic and Geomorphological Investigation of the Tertiary Volcanic Sequence of North Eastern New South Wales. Masters Thesis, Southern Cross University.
*Duggan, P.B., Mason, D.R. 1978. Stratigraphy of the Lamington Volcanics in Far Northeastern New South Wales. Australian Journal of Earth Sciences V25.
*Howden, S. 2009. An Evaluation of Mafic Extrusives Spatially Assoicated with the South-Western Aspect of the Tweed Shield Volcano, BSc(Hons.) thesis, University of New England, Armidale.
*Smith, J.V. , Houston, E.C. 1995. Structure of lava flows of the Nimbin Rhyolite, northeast New South Wales. Australian Journal of Earth Sciences V42(1) p69-74.

Why lava is unable to cross the state border!

Nimbin Rhyolite (front), Mt. Warning (right), Binna Burra Rhyolite (distant)

It is interesting to see how being north or south of the Queensland border influences so many things. North of the border you can get cheaper car registration, get away from NSW politics, follow a worst rugby league teams, follow the best soccer teams, see narrower gauge railways (which even frequently have trains on them!) and find that even the rocks have changed.

Actually, the rocks have not changed but for some reason I cannot fathom (like most of the other points raised above) the rocks often have different names but many have the same ones. Rocks of the Mesozoic Clarence-Moreton basin have the same names, rocks of the Palaeozoic basement have the same names, but rocks of the Lamington Volcanics are named differently. You can stand on the Lismore Basalt and take one step into Queensland and you are on the Beechmont Basalt. Suffice to say it can be confusing. So, based on Duggan and Mason (1978) here is a table to show what the rocks units in the Lamington Volcanics are called in either state:

New South Wales  - Queensland

Kyogle Basalt - Albert Basalt

Homeleigh Agglomerate Member - Mount Gillies Rhyolite

Lismore Basalt - Beechmont Basalt

Nimbin Rhyolite - Binna Burra Rhyolite

Blue Knob Basalt - Hobwee Basalt

I note that there is some other rock units that are named differently in NSW and Queensland for example McElroy (1969) shows that such as the Evans Head Coal Measures, Ipswich coal measures and Red Cliff Coal Measures (Parts of the Ipswich Basin) are equivalent to each other, but these are separated by different rocks and so occur in distinctly different geographical locations. But as far as I am aware the Lamington volcanics are the most obvious example where an invisible dotted line representing the state border can name one half of the same rock, formed in the same way, at the same time, at the same outcrop something different.

It is also important to note that stratigraphy is often refined once more is known about rock units. A good example is that some authors such as Cotter 1998 dispute the existence of the Homeleigh Agglomerate Member which is considered part of the Nimbin Rhyolite. Also the Mount Gilllies Rhyolite has been renamed the Mount Gillies Volcanics, Therefore a different unit called the Georgica Rhyolite would be an equivalent of the Mount Gillies Volcanics.

I know I’ve said it elsewhere, but geology is not usually too difficult. The worst part is the nomenclature. I think this is a good example. What do you think?

References/bibliography:

*Cotter, S. 1998. A geochemical, Palaeomagnetic and Geomorphological Investigation of the Tertiary Volcanic Sequence of North Eastern New South Wales, Masters Thesis, Southern Cross University. 
*Duggan, P.B., Mason, D.R. 1978. Stratigraphy of the Lamington Volcanics in Far Northeastern New South Wales. Australian Journal of Earth Sciences V25.
*McElroy, C.T. 1969. The Clarence-Moreton Basin in New South Wales. In Packham, G.H.(ed) - The geology of New South Wales. Geological Society of Australia. Journal V16.

What's the difference between the basalts?

A vesicular (air bubbles) example of Alstonville Basalt

There are three recognized Cenozoic aged "basaltic" geological units in the area between the Queensland border and Evans Head. These were first classified by Duggan and Mason (1978) and are the Blue Knob, Kyogle and Lismore Basalts. These 'basalts' are all part of the Lamington Volcanics.C otter (1998) has also proposed a new unit known as the Alstonville Basalt and included these in the Lamington Volcanics too but the information by Cotter was 'lost' until recently.  All four of these units are described below from oldest to youngest.

Alstonville Basalt
This is a new unit proposed by Cotter (1998), dating by this author gives a date of around 41 million years. This means that the Alstonville Basalt is too old to have formed through the same mechanism as the Tweed Volcano/Mount Warning basalts that are discussed below. No model of formation has been proposed but other research Vickery et al (2007) from the basalts of the New England tablelands area has proposed that a basalt of similar composition and age known as the Maybole Volcanics formed during rifting associated with the opening of the Tasman Sea. So this mechanism may be appropriate for the Alstonville Basalt too.
The Alstonville Basalt is actually similar in composition to the Kyogle Basalt in that it consists mainly of basalt and andesite called hawaiite which means that there is no mineral quartz in the rock but the mineral olivine is commonly found instead.

Kyogle Basalt
In Queensland the Kyogle Basalt is called the Albert Basalt. Wellman and McDougall 1974 give the age of the Albert Basalt at 22.5 million years (and accordingly the Kyogle Basalt would be the same age). The origin of this unit is regarded as the Focal Peak volcano which is situated today around Mount Barney. The Kygole Basalt predominately consists of a basalt called hawaiite with minor basanite and alkaline olivine basalt (basalts which are silica poor with no quartz in the rock but some olivine). Rarely tholeiitic basalt also occurs (basalt with some quartz which has crystallized in a specific geochemical pattern). The minerals that make up the smallest crystals in the rock (the groundmass) generally have a green colour giving the Kyogle Basalt a green tinge which often helps with identification in the field.

As the Australian Plate drifted over a hot spot in the mantle a chain of volcanoes was formed with the oldest situated in Queensland and the youngest (and still active or just dormant) volcanoes situated in Victoria and out in the Southern Ocean. The Kyogle Basalt represents the commencement of hot spot volcanism (i.e. the beginning of the Tweed and Focal Peak volcanoes) in the region.

Lismore Basalt
The Lismore Basalt is called the Beechmont Basalt in Queensland which has been given an age of between 22.6 to 22.9 million years. In some areas Duggan and Mason (1978) have mapped the Lismore Basalt as directly overlying the Kyogle Basalt. However, it is important to note that in the field the distinction between the two units can be difficult at times. The Lismore basalts are mainly tholeiitic in nature (usually contain a little bit of quartz and no olivine). The distribution of the Lismore Basalt is greatest for all the units of the Lamington Volcanics in NSW with the unit exposed over an area of greater than 3 000 square kilometres. It is the major eruptive unit originating from the Tweed Shield Volcano which is centred at present day Mount Warning.

Blue Knob Basalt
There is actually very little difference between the Blue Knob and Lismore Basalts except that the two units are separated by units of rhyolite known as the Nimbin Rhyolite. Some authors such as Duggan and Houston (1978) and Smith and Houston (1995) have even suggested that they represent continuing sporadic eruptions of the Lismore Basalt during the period of eruptions of the Nimbin Rhyolite. The basalts outcrop on top of or inter-collated with the Nimbin Rhyolite and may actually represent a continuity of occasional basalt lava eruptions while the rhyolite lavas were erupted. However, the Blue Knob Basalt represents the final preserved eruptions known of the Tweed Volcano.

In Queensland the Blue Knob Basalt is called the Hobwee Basalt.

Note: Now, if you are a little bamboozled by all the weird names of the basalts and how basalts can appear to be identical and called something else in a different location (especially given state borders) please keep with me because in the near future I will do a post that explains the difference. I'll also have to find some sources online to explain how basalts are different from each other (and how to tell that difference in the field). In the mean time the glossary may provide some assistance.


References/Bibliography:

*Cotter, S. 1998. A Geochemical, Palaeomagnetic and Geomorphological Investigation of the Tertiary Volcanic Sequence of North Eastern New South Wales. Masters Thesis, Southern Cross University.
*Duggan, P.B., Mason, D.R. 1978. Stratigraphy of the Lamington Volcanics in Far Northeastern New South Wales. Australian Journal of Earth Sciences V25.
*Smith, J. V., Houston, E.C. 1995. Structure of lava flows of the Nimbin Rhyolite, northeast New South Wales. Australian Journal of Earth Sciences v42.
*Vickery, N. M., Dawson, M.W., Sivell, W.J., Malloch, K.R., Dunlap, W.J. 2007. Cainozoic igneous rocks in the Bingara to Inverell area, northeastern New South Wales. Geological Survey of New South Wales Quarterly Notes v123.
*Wellman, P. & McDougall, I. 1974. Potassium-argon Dates on the Cainozoic Volcanic Rocks of New South Wales. Journal of the Geological Society of Australia v21.